Understanding the molecular basis for why viruses cause disease is an essential requirement for future development of antiviral therapeutics. While we know a great deal about how viruses are recognized and the various ways in which they are combated, there are still vast areas of cellular antiviral immunity that remain unknown. This research proposal focuses on one such area. The central question being addressed in this proposal is whether small RNAs, or components of the mammalian small RNA machinery, contribute to the cellular response to infection. The importance of this question and the need to address it experimentally can be justified through extensive preliminary data that suggest a novel means to combat virus infection is present in mammalian cells but awaits characterization. In an effort to understand the interplay between the small RNA pathway of mammals and the response to virus infection we have discovered the potency by which the RNA induced silencing complex (RISC) can inhibit virus replication, isolated the first mammalian virus protein which inhibits RISC function, and found that Drosha, a key player in small RNA biogenesis, actively translocates to the cytoplasm during RNA virus infection. Taken together, we believe this research provides compelling evidence for an unappreciated small RNA-mediated antiviral response in mammals. Here we propose to use the virus antagonist to RISC (called VP55) to define the small RNA response to infection in vivo as well as characterize how Drosha participates in this antiviral activity. The overall objective of this proposal is to delineate howthe small RNA pathway contributes to our response to virus infection.